1. Field of the Invention
The present invention relates to a process of sharpening tapered silicon structures. Specifically, the present invention relates to a process that is useful for sharpening tapered silicon structures, such as field emitters, or field emission tips, on a substrate after circuit traces or other metal layers or structures have been formed on the substrate. The present invention also relates to a method of fabricating sharply pointed or tapered structures from substrates such as silicon wafer, silicon-on-insulator (SOI), silicon-on glass (SOG), and silicon-on-sapphire (SOS).
2. Background of Related Art
Tapered structures have long been employed as field emitters in electronic display devices. Due to the ever-improving electron emission characteristics of silicon field emitters, and since silicon field emitters are relatively inexpensive to fabricate, their use in electronic display devices is ever-increasing. The ability of silicon field emitters to emit electrons is partially dependent upon the sharpness of the tips, or apices, thereof. Sharply tipped field emitters require less energy than more bluntly tipped field emitters to achieve a desired degree of electron emission. Accordingly, the improvement of silicon field emitters is due, in part, to state of the art techniques for fabricating such structures, with which techniques field emitters of ever-increasing sharpness may be fabricated.
Conventional processes for fabricating silicon field emitters typically include a mask and etch of a substrate in order to define a silicon field emitter. The silicon field emitter may then be sharpened by thermal oxidation of an exposed surface of the silicon field emitter, which typically occurs at a temperature exceeding 900xc2x0 C., and the subsequent removal of the oxide layer from the field emitter. Subsequently, associated structures may be fabricated on the substrate and assembled therewith in order to manufacture a field emission display device.
Many state of the art silicon field emitter fabrication processes, however, are somewhat undesirable in that some field emitter tips lack a desirable level of sharpness (i.e., are xe2x80x9cbluntxe2x80x9d), which typically increases the amount of voltage that is required in order for the field emitter to properly function.
The increased voltage requirements of blunt field emitters may cause them to fail to turn xe2x80x9conxe2x80x9d or to xe2x80x9chardly turn xe2x80x98onxe2x80x99xe2x80x9d. In order to function properly, field emitters that hardly turn xe2x80x9conxe2x80x9d require a voltage that exceeds a desired, or xe2x80x9cexpectingxe2x80x9d, operating voltage range. In contrast, properly functioning field emitters, which typically include sharp tips, turn xe2x80x9conxe2x80x9d, and therefore function properly, when a voltage within the expecting voltage range is applied thereto. The failure of a field emitter to turn xe2x80x9conxe2x80x9d within the expecting voltage range may result in the failure of a field emission display including such a field emitter. xe2x80x9cFailedxe2x80x9d field emission display devices are typically scrapped or discarded, which decreases product yield and results in increased production costs.
For the same reasons described above, the variable voltage requirements created by nonuniformities in the sharpness of the field emitters of a field emission display device may create brightness nonuniformities on a display screen that is illuminated thereby, even in devices which include silicon field emitters that turn xe2x80x9conxe2x80x9d within the expecting voltage range. While sharper field emitters will brightly illuminate their corresponding areas of a display screen, areas of the display screen that are illuminated by blunter field emitters will be relatively dim. Thus, although a field emission display device which includes blunt field emitters may not fail production testing, sharpness nonuniformities may cause unacceptable brightness nonuniformities on a finished display screen.
Techniques for fabricating field emission displays with silicon emitters of substantially uniform sharpness typically include repetitive thermal oxidation of the exposed surface of the field emitters and the subsequent removal of the oxide layer from the field emitters. Due to the high temperatures that are typically utilized in such thermal oxidation processes, however, relatively thick oxide layers are formed on the field emitters. Thus, it may be difficult to control the sharpness of the tips of the field emitters.
An exemplary state of the art process for fabricating tapered silicon structures, such as silicon field emitters, is disclosed in U.S. Pat. No. 5,201,992 (the xe2x80x9c""992 patentxe2x80x9d), which issued to Robert B. Marcus et al. on Apr. 13, 1993. The process of the ""992 patent includes defining protuberances by conventional mask and etch techniques and thermally oxidizing the exposed surface of each of the protuberances in a dry-oxygen environment at a temperature of between about 900xc2x0 C. and 1050xc2x0 C. The oxide layer is then removed from the protuberances by conventional etch techniques in order to define the tapered structures. Thermal oxidation may be repeated to enhance the sharpness of the apices of the tapered structures. Following sharpening of each of the tapered structures, the sharpness of the apices may subsequently be decreased by thermally oxidizing same in either a wet or dry oxygen environment at a temperature exceeding 1050xc2x0 C.
While the process of the ""992 patent fabricates tapered silicon structures with sharp apices, the process cannot be employed on finished structures which include tapered silicon structures, such as field emission display arrays including circuit traces or other metal structures thereon. Thus, the process of the ""992 patent is not useful for reworking finished field emission display arrays in order to decrease failure rates thereof or otherwise improving such finished field emission display arrays.
Moreover, with reference to FIG. 1, the repeated thermal oxidation of silicon field emitters is somewhat undesirable from the standpoint that the typically high temperatures that are utilized in such oxidation processes may create crystalline defects, which are indicated by arrows, in the silicon field emitter, such as point, line (e.g., slip, straight dislocations, dislocation loops, etc.), area, volume, or other crystalline defects. These crystalline defects may also increase the voltage requirement of the silicon field emitter.
Many conventional thermal oxidation processes that are employed to fabricate tapered silicon structures are further undesirable from the standpoint that the oxide layers formed thereby are relatively thick (e.g., on the order of hundreds of angstroms). Thus, as such an oxide layer is subsequently removed from the silicon structure, it may be difficult to control the sharpness of the silicon structure. Such conventional thermal oxidation processes form thick oxide layers due, in part, to the small process windows of such processes. Many conventional thermal oxidation processes may also damage the substrate which underlies the sharpened silicon structure, such as the glass of silicon-onglass substrates that are typically employed in manufacturing displays that are larger than the currently available silicon wafers.
Conventionally, the failure rates of field emission display devices have been relatively high. Although field emitters of substantially uniform sharpness may be fabricated by some known processes, field emission display devices are typically not tested until after circuit traces and other metal structures associated therewith have been fabricated. Thus, conventional thermal oxidation processes cannot be employed to further sharpen silicon field emitters, as the high temperatures of such processes may damage any metal structures that have been fabricated on the substrate upon which the field emitters are located.
Thus, a process is needed for reworking failed and marginally functional silicon field emitters without introducing crystalline defects therein and without damaging the substrate or any circuitry associated with the silicon field emitters. A process for fabricating sharply pointed or tapered silicon structures, such as sharp silicon field emitters, with substantially uniform sharpness, and without introducing additional crystalline defects therein is also needed.
The method of the present invention addresses each of the foregoing needs.
A first embodiment of the method of the present invention comprises sharpening a tapered or pointed silicon structure, such as a silicon field emitter of an existing field emission display. Sharpening a tapered or pointed silicon structure, such as a silicon field emitter, includes oxidizing an exposed surface of same at a relatively low, even extremely low, temperature and removing the oxide layer. The oxidization of the exposed surfaces of the silicon structure and the subsequent removal of the oxide layer therefrom may be repeated to further sharpen the silicon structure.
The tapered or pointed silicon structure, such as a silicon field emitter, may be oxidized at a temperature that will not damage any circuit traces or other metal layers or structures that are associated with the substrate upon which the field emitter is located. Thus, oxidation preferably occurs at a temperature that is less than the melting point of each of the metal structures that are associated with the substrate. An exemplary oxidation temperature is room temperature, which is typically in the range of about 22xc2x0 C. to about 27xc2x0 C.
Oxidation processes which have relatively large process windows may be employed in the first embodiment of the inventive method. Such oxidation processes facilitate the formation of a relatively thin oxide layer, such as on the order of tens of angstroms, on the field emitter.
The oxide layer formed on the silicon structure (e.g., field emitter) is removed by etching techniques that are known in the art. Preferably, an etching technique which removes silicon oxide from a silicon substrate without substantially etching the silicon substrate (i.e., a process which utilizes an etchant that is selective for silicon oxide over silicon) is employed to remove the oxide layer from the silicon field emitter.
A second embodiment of the method of the present invention comprises a method of fabricating a tapered silicon structure, such as a field emitter of a field emission display device. The second embodiment is particularly useful for defining tapered silicon structures from a silicon layer of a silicon-on-glass substrate. The second embodiment of the inventive method includes patterning a silicon layer to define a rough silicon structure therefrom, oxidizing the rough silicon structure to form a first oxide layer thereon at a low temperature, re-etching the oxide layer to define a silicon structure, oxidizing the exposed surface of the silicon structure at a low temperature to form a second oxide layer thereon, and removing the second oxide layer to define a finished silicon structure. Low temperature oxidation and re-etching may be repeated in order to form a sharper taper.
Techniques that are known in the art, such as mask and etch processes, may be employed to pattern the silicon layer in order to define the rough silicon structure. Subsequent oxidation and re-etching of the rough silicon structure may also be performed by techniques that are known in the art.
Preferably, low temperature oxidation of the silicon structure forms a relatively thin oxide layer on the exposed surfaces thereof, on the order of tens of angstroms. Thus, the low temperature oxidation techniques that are useful in the inventive method have relatively large process windows, which allow for precise control over the thickness of the second oxide layer that is formed on the silicon structure, relative to the process windows of conventional thermal oxidation processes.
The second oxide layer is removed from the finished structure by etching techniques that are known in the art. Preferably, an etching technique which removes silicon oxide from a silicon substrate without substantially etching the silicon substrate (i.e., a process which utilizes an etchant that is selective for silicon oxide over silicon) is employed to remove the oxide layer from the silicon structure in order to define the finished silicon structure.
Other advantages of the present invention will become apparent through a consideration of the ensuing description of the invention, the accompanying drawings, and the appended claims.